This invention relates to emergency evacuation equipment for aircraft. In particular, this invention relates to an inflation device for inflating an inflatable aircraft evacuation slide or other inflatable device.
The requirement for reliably evacuating airline passengers in the event of an emergency is well known. Emergencies at take-off and landing often demand swift removal of the passengers from the aircraft because of the potential for injuries from fire, explosion, or sinking in water. A conventional method of quickly evacuating a large number of passengers from an aircraft is to provide multiple emergency exits, each of which is equipped with an inflatable evacuation slide. Current state of the art emergency evacuation slide systems comprise an inflatable evacuation slide which is stored in an uninflated folded state together with a source of inflation gas. The source of inflation gas typically comprises a gas generator, stored compressed gas, or a combination thereof. Pyrotechnic gas generators have an advantage in that they are small, lightweight, and produce a high volume of gas, however, the high temperature gas produced by a gas generator alone can cause numerous problems including sagging of the evacuation slide as the inflation gas cools and, in some cases, melting of the fabric out of which the inflation slide if fabricated. Use of stored compressed gas by itself, although simple, implicates a weight penalty that must be paid for carrying a pressure vessel having sufficient capacity (in terms of volume and pressure) to inflate the evacuation slide over the wide operational temperature range specified for such slides. Additionally, where only a compressed gas is used to inflate the evacuation slide, a large drop in temperature occurs as the gases expand, often causing ice to form, which can block the flow of gas. Accordingly, state-of-the-art emergency evacuation slide evacuation systems typically comprise a hybrid inflator, which utilizes a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas, as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel.
To further augment the volume of gas delivered to the evacuation slide, many evacuation systems utilize aspirators such as that disclosed in U.S. Pat. No. 4,368,009 to Heimovics, et al. As the compressed gas flows through the aspirator, a venturi is produced that causes the aspirator to pump about two to three times as much gas as is supplied by the compressed gas source alone.
Despite these advances, there still exist problems due to the wide ambient temperature range over which these inflation systems are required to operate, typically from −65° F. to +165° F. The amount of gas available must be enough to pressurize the evacuation slide at the coldest temperature. However, because of the relationship between pressure and temperature within a fixed volume, as the ambient temperature rises above the minimum, the pressure within the pressure vessel rises proportionately. Accordingly, in current state-of-the-art hybrid inflation systems, the storage vessel must be capable of withstanding the pressure of the compressed gas at 165° F. Not only this, but the storage vessel must withstand the overpressure at 165° F. caused by the pyrotechnic gas generator, even though use of the pyrotechnic gas generator at 165° F. causes the inflation system to produce substantially more gas than is necessary to inflate the evacuation slide. Additionally, the substantial overpressure caused at high temperature further increases the weight of the system, because additional Pressure Relief Valves (PRV's) must be incorporated into the evacuation slide to vent the excess gases.
Accordingly, what is needed is a system for tailoring initiation of the pyrotechnic gas generator in a hybrid compressed gas/pyrotechnic inflation device to prevent overpressurization of the compressed gas pressure vessel over a wide range of temperatures.